High-temperature spin dynamics in the Heisenberg chain: Magnon propagation and emerging KPZ-scaling in the zero-magnetization limit

TL;DR

This paper investigates the zero-magnetization limit of the Heisenberg chain, revealing a crossover from ballistic to KPZ-like sub-ballistic spin dynamics, with implications for understanding quantum integrable systems at finite temperatures.

Contribution

It provides the first detailed numerical analysis showing KPZ scaling in the spin dynamics of the Heisenberg chain at zero magnetization.

Findings

At zero magnetization, spin dynamics exhibit KPZ scaling.

Propagating magnon peaks diminish as magnetization approaches zero.

Central peak dynamics become sub-ballistic with a dynamical exponent near 3/2.

Abstract

The large-scale dynamics of quantum integrable systems is often dominated by ballistic modes due to the existence of stable quasi-particles. We here consider as an archetypical example for such a system the spin-$\frac{1}{2}$ XXX Heisenberg chain that features magnons and their bound states. An interesting question, which we here investigate numerically, arises with respect to the fate of ballistic modes at finite temperatures in the limit of zero magnetization $m{=}0$. At a finite magnetization density $m$, the spin autocorrelation function $\Pi(x,t)$ (at high temperatures) typically exhibits a trimodal behavior with left- and right-moving quasi-particle modes and a broad center peak with slower dynamics. The broadening of the fastest propagating modes exhibits a sub-diffusive $t^{1/3}$ scaling at large magnetization densities, $m {\rightarrow} \frac{1}{2}$, familiar from non-interacting models; it crosses over into a diffusive scaling $t^{1/2}$ upon decreasing the magnetization to smaller values. The behavior of the center peak appears to exhibit a crossover from transient super-diffusion to ballistic relaxation at long times. In the limit $m{\to}0$, the weight carried by the propagating peaks tends to zero; the residual dynamics is carried only by the central peak; it is sub-ballistic and characterized by a dynamical exponent $z$ close to the value $\frac{3}{2}$ familiar from Kardar-Parisi-Zhang (KPZ) scaling. We confirm, employing elaborate finite-time extrapolations, that the spatial scaling of the correlator $\Pi$ is in excellent agreement with KPZ-type behavior and analyze the corresponding corrections.